Letters sent to President Barack Obama and Sen. Roger Wicker that have preliminarily tested positive for the deadly poison ricin are related and were both sent from Memphis, Tenn., the FBI said Wednesday. A Missouri senator said police have a suspect in mind.

The FBI says that the letters sent to Obama and Sen. Roger Wicker, R-Miss., both say: "To see a wrong and not expose it, is to become a silent partner to its continuance," and are signed, "I am KC and I approve this message," according to an intelligence bulletin obtained by the Associated Press.

The letters were postmarked on April 8. Sen. Claire McCaskill, D-Mo., said Tuesday that police suspect a person who "writes a lot of letters to members," the Associated Press reported. 

Ricin is a poison found in castor beans, and can be in the form of a powder, mist, or a pellet, according to the definition on the Center for Disease Control and Prevention website.

U.S. Capitol police are also investigating at least three suspicious packages in Senate office buildings, according to the Associated Press.

In a statement issued Wednesday, the FBI said they do not believe there is a connection between the tainted letters and Monday's bombings at the Boston Marathon, CNN News reported.